The present invention is directed to the configurations of an optical communication system in which a plurality of optical line units share an optical transmission line, and the methods of operating the same. In particular, the present invention relates to the configuration of an optical communication system excellent in system expandability, such as the capability to extend the transmission distance of the system and the capability to increase the number of accommodated subscribers, and the method of operating the same.
With an increasing demand for communications utilizing a broadband, the access line for users is shifting to a large-capacity access line using an optical fiber, in place of the access techniques based on the telephone line, such as DSL (Digital Subscriber Line). Today, as an access network, a PON (Passive Optical Network) system (hereinafter, may be simply referred to as a PON, or an optical passive network system or passive optical network system) is often used from the viewpoints of the line installation cost and maintenance cost. This PON is now being standardized by various standardization organizations, including International Telecommunication Union Standardization Sector (hereinafter, referred to as ITU-T). One example is G-PON (Gigabit PON) standardized by ITU-T Recommendation G. 984.3, etc.
The PON is a system, wherein an optical signal is transmitted and received by branching and multiplexing the optical signal between an optical line terminal (hereinafter, referred to as an OLT or a master station) and a plurality of optical network units (hereinafter, referred to as ONUs or slave stations) using an optical fiber and an optical splitter. Because the performance, such as the attenuation amount of an optical signal passing through the optical fiber, is limited by the transmission performance of the optical fiber to be used and the number of optical branches in the optical splitter, the PON has a limitation in the communication distance between the OLT and the ONU. For example, in the case of G-PON, the communication distance is set to 20 km at the maximum and the number of branches by the optical splitter (the number of ONUs which can be coupled to the OLT) is set to 64 at the maximum.
With an increase in the opportunities for subscribers (communication network users) in households to access the Internet and carry out communication for the purpose of information collection or social life, there is a need to increase the number of communication network facilities, such as the number of access networks which couple subscribers to the communication network. For this reason, a method of additionally introducing a PON itself used in the access network, i.e., adding an OLT, or a method of expanding the number of ONUs which an OLT of a PON accommodates may be considered. In a PON, usually, an OLT performs a complicated system-control, such as a bandwidth control, or the management of accommodated ONUs. Here, the OLT is much more expensive than the ONU. Moreover, the cost for newly installing an optical fiber leads to a major expenditure to a carrier. Therefore, it is usually preferable to increase the number of accommodated ONUs per an OLT rather than to additionally install an OLT.
The introduction of a reach expander (hereinafter, referred to as an RE), which is used for extending the communication distance of an optical fiber or increasing the number of branches, into the existing PON is under study. The RE is appropriately installed within an optical signal communication interval between an OLT and an ONU and this RE is controlled from the OLT, thereby achieving an extension of the communication distance of an optical fiber or an increase in the number of branches. Note that a control protocol has been standardized by ITU-T based on a proposal of using the OMCI (ONU Management Control Interface) which is the existing ONU control protocol (see ITU-T Recommendation G. 984.4).